1. Field of the Invention
The present invention relates to a calibration method for inertial drive actuator, an inertial drive actuator device, and a method of calculating a position of a moving body.
2. Description of the Related Art
FIG. 16 is a side-view depicting the structure of a conventional actuator 920. As shown in FIG. 16, the actuator 920 includes a piezoelectric element 911, which is a type of electromechanical converting element, a drive shaft 912, a moving body 913 that is friction-coupled to the drive shaft 912, and a frame 914 of the actuator 920. One end of the piezoelectric element 911 is fixed to the frame 914 while the other end of the piezoelectric element 911 is fixed to the drive shaft 912.
A detecting member 921 that is fixed to the frame 914 constitutes a fixed electrode for detecting a position of the moving body 913 based on an electrostatic capacitance. The detecting member 921 is disposed parallel to a direction of movement of the moving body 913 in a contactless manner. The drive shaft 912, the moving body 913, and the detecting member (fixed electrode) 921 are made of a conductive material. The surface of the detecting member 921 that opposes the moving body 913 constitutes a detecting member 921. The detecting member 921 and the moving body 913 are arranged with a gap D therebetween and they constitute a capacitor having an electrostatic capacitance C.
FIG. 17 is a plan-view depicting the structure of the detecting member 921 and the relationship between the detecting member 921 and the moving body 913. As shown in FIG. 17, the detecting member 921 includes a first electrode 921a and a second electrode 921b arranged on an insulation member 921p. Each of the first electrode 921a and the second electrode 921b has a shape of a right-angled triangle. The first electrode 921a and the second electrode 921b are arranged in such a way that their oblique sides are adjacent to each other. A driving signal output from a driving circuit 918 (see FIG. 16) is applied to the piezoelectric element 911 and also to the moving body 913 via the drive shaft 912.
As in an exemplary state shown in FIG. 17, the moving body 913 and the first electrode 921a face each other and are coupled by electrostatic-capacitive coupling. Similarly, the moving body 913 and the second electrode 921b face each other and they are coupled by an electrostatic-capacitive coupling. As a result, the driving signal applied to the moving body 913 flows toward the first electrode 921a and the second electrode 921b. A current i flowing toward the first electrode 921a and the second electrode 921b is detected by a detecting circuit 919 the value of the current i is input into a control circuit 917.
As an example, a case is explained here in which the moving body 913 moves in the direction of an arrow a (see FIG. 17) from the first electrode 921a toward the second electrode 921b. Because of the movement of the moving body 913, while on one hand an opposing electrode surface area between the moving body 913 and the first electrode 921a decreases gradually leading to a gradual decrease in an electrostatic capacitance Ca between the two, on the other hand an opposing electrode surface area between the moving body 913 and the second electrode 921b increases gradually leading to a gradual increase in an electrostatic capacitance Cb between the two. Consequently, as the moving body 913 moves, a current is flowing from the moving body 913 to the first electrode 921a decreases gradually, and a current ib flowing from the moving body 913 to the second electrode 921b increases gradually.
On the other hand, when the moving body 913 moves in the opposite direction of the arrow a, from the second electrode 921b toward the first electrode 921a, while on one hand the opposing electrode surface area between the moving body 913 and the first electrode 921a increases gradually leading to a gradual increase in the electrostatic capacitance Ca between the two, on the other hand the opposing electrode surface area between the moving body 913 and the second electrode 921b decreases gradually leading to a gradual decrease in the electrostatic capacitance Cb between the two. Consequently, as the moving body 913 moves, the current ia flowing from the moving body 913 to the first electrode 921a increases gradually, and the current ib flowing from the moving body 913 to the second electrode 921b decreases gradually.
Thus, the position of the moving body 913 in relation to the detecting member 921 can be determined by comparing the amounts of the currents ia and ib that increase and decrease with the movement of the moving body 913. In addition, the direction of movement of the moving body 913 can be determined based on whether the currents ia and ib increase or decrease.
Such an actuator is disclosed, for example, in Japanese Patent Application Laid-open No. 2003-185406.
However, with use, due to factors such as humidity, temperature, gravity, and aging, the electrostatic capacitance of the actuator 920 tends to differ from the initial value of the actuator 920 detected at the time of its assembly. The electrostatic capacitance between the moving body 913 and the detecting member 921 is being measured in this case. When the electrostatic capacitance is large, an electric resistance decreases, leading to an increase of a voltage and a current of a received detection signal. This may cause an error, a false decision, or a malfunctioning in an A/D converter. Conversely, when the electrostatic capacitance is small, the electric resistance increases, leading to a decrease of the voltage and the current of the received signal. In this situation, because the A/D converter cannot be efficiently used, a position detection precision may degrade.